The sea is the connected body of salt water that covers 70 percent of the Earth's surface. The sea is important in
moderating the Earth's climate, in providing food and oxygen, in its enormous diversity of life, and for navigation. The study of the sea is called oceanography. The sea has been travelled and explored since ancient times, but its scientific study dates broadly from the voyages of Captain James Cook to explore the Pacific Ocean between 1768 and 1779. Seawater is characteristically salty, containing as its main salt sodium chloride but also chlorides of potassium and magnesium, alongside many other chemical elements in a composition that hardly varies across the world's oceans. However the salinity varies quite
widely, being higher at the surface in warm waters, and less in the depths and near large rivers.
The sea surface is subject to waves caused by winds. Waves decelerate and increase in height as they enter shallow water, becoming tall and unstable, and breaking into foam on the shore. Tsunamis are caused by submarine earthquakes or landslides and may be barely noticeable out at sea but can be violently destructive on shore. Winds create currents as well as waves through friction with the water, setting up slow but stable circulations of water throughout the sea. The directions of the circulation are governed by several factors including the shapes of the continents and the rotation of the earth. Complex deep sea currents known as the global conveyor belt carries cold water from near the Poles to every ocean. Large-scale movement of seawater is caused also by the tide, the twice-daily rhythm of the gravitational pull exerted by the Moon and, to a lesser extent, by the Sun on the Earth. Tides may have a very high range in bays or estuaries such as the Bay of Fundy which funnel tidal flows into narrow channels.
The sea is home to all the major groups of living organisms including bacteria, protists, algae, plants, fungi, and animals. It is believed to be where life first began, as well as where many of the major groups evolved. The sea contains a wide range of habitats and ecosystems, ranging vertically from the surface and the shoreline to the enormous depths and pressures of the cold, dark benthic zone, and in latitude from the waters under the Arctic ice to the colourful diversity of coral reefs in tropical waters.
Overview[]
The sea covers fully 70 percent of the Earth's surface with liquid water. Seen from space, our planet appears as a
"blue marble" of various forms of water: salty oceans, sea ice, clouds. The science fiction author Arthur C. Clarke once suggested that "Earth" should have been named "Ocean" as the sea is its dominant feature, both critically important to life on earth, and recently strongly affected by human activities such as overfishing and pollution.
The sea is from one point of view the World Ocean, the interconnected system of all the Earth's oceanic waters. About 97.2 percent of the Earth's water is found in the seas, some 326 million cubic miles of salty water. Of the rest, 2.15 percent is accounted for by ice in glaciers, surface deposits and sea ice and 0.65% is in the form of vapour or liquid fresh water in lakes, rivers, the ground and air.